Emmanuel Planel

Tau filament formation and associative memory deficit in aged mice expressing mutant (R406W) human tau

The R406W tau mutation found in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) causes a hereditary tauopathy clinically resembling Alzheimers disease. Expression of modest levels of the longest human tau isoform with this mutation under the control of the α-calcium–calmodulin-dependent kinase-II promoter in transgenic (Tg) mice resulted in the development of congophilic hyperphosphorylated tau inclusions in forebrain neurons. These inclusions appeared as early as 18 months of age. As with human cases, tau inclusions were composed of both mutant and endogenous wild-type tau, and were associated with microtubule disruption and flame-shaped transformations of the affected neurons. Straight tau filaments were recovered from Sarkosyl-insoluble fractions from only the aged Tg brains. Behaviorally, aged Tg mice had associative memory impairment without obvious sensorimotor deficits. Therefore, these mice that exhibit a phenotype mimicking R406W FTDP-17 provide an animal model for investigating the adverse properties associated with this mutation, which might potentially recapitulate some etiological events in Alzheimers disease.

Alterations in Glucose Metabolism Induce Hypothermia Leading to Tau Hyperphosphorylation through Differential Inhibition of Kinase and Phosphatase Activities: Implications for Alzheimer's Disease

Alzheimers disease (AD) brains contain neurofibrillary tangles (NFTs) composed of abnormally hyperphosphorylated tau protein. Regional reductions in cerebral glucose metabolism correlating to NFT densities have been reported in AD brains. Assuming that reduced glucose metabolism might cause abnormal tau hyperphosphorylation, we induced in vivo alterations of glucose metabolism in mice by starvation or intraperitoneal injections of either insulin or deoxyglucose. We found that the treatments led to abnormal tau hyperphosphorylation with patterns resembling those in early AD brains and also resulted in hypothermia. Surprisingly, tau hyperphosphorylation could be traced down to a differential effect of low temperatures on kinase and phosphatase activities. These data indicate that abnormal tau hyperphosphorylation is associated with altered glucose metabolism through hypothermia. Our results imply that serine-threonine protein phosphatase 2A plays a major role in regulating tau phosphorylation in the adult brain and provide in vivo evidence for its crucial role in abnormal tau hyperphosphorylation in AD.

Retromer deficiency observed in Alzheimer's disease causes hippocampal dysfunction, neurodegeneration, and Aβ accumulation

Although deficiencies in the retromer sorting pathway have been linked to late-onset Alzheimers disease, whether these deficiencies underlie the disease remains unknown. Here we characterized two genetically modified animal models to test separate but related questions about the effects that retromer deficiency has on the brain. First, testing for cognitive defects, we investigated retromer-deficient mice and found that they develop hippocampal-dependent memory and synaptic dysfunction, which was associated with elevations in endogenous Aβ peptide. Second, testing for neurodegeneration and amyloid deposits, we investigated retromer-deficient flies expressing human wild-type amyloid precursor protein (APP) and human β-site APP-cleaving enzyme (BACE) and found that they develop neuronal loss and human Aβ aggregates. By recapitulating features of the disease, these animal models suggest that retromer deficiency observed in late-onset Alzheimers disease can contribute to disease pathogenesis.

Aberrant tau phosphorylation by glycogen synthase kinase-3β and JNK3 induces oligomeric tau fibrils in COS-7 cells

Neurofibrillary tangles (NFTs) are found in a wide range of neurodegenerative disorders, including Alzheimers disease. The major component of NFTs is aberrantly hyperphosphorylated microtubule-associated protein tau. Because appropriate in vivo models have been lacking, the role of tau phosphorylation in NFTs formation has remained elusive. Here, we describe a new model in which adenovirus-mediated gene expression of tau, ΔMEKK, JNK3, and GSK-3β in COS-7 cells produces most of the pathological phosphorylation epitopes of tau including AT100. Furthermore, this co-expression resulted in the formation of tau aggregates having short fibrils that were detergent-insoluble and Thioflavin-S-reactive. These results suggest that aberrant tau phosphorylation by the combination of these kinases may be involved in “pretangle,” oligomeric tau fibril formation in vivo.

U-box protein carboxyl terminus of Hsc70-interacting protein (CHIP) mediates poly-ubiquitylation preferentially on four-repeat Tau and is involved in neurodegeneration of tauopathy.

Neurofibrillary tangles (NFTs), which are composed of hyperphosphorylated and ubiquitylated tau, are exhibited at regions where neuronal loss occurs in neurodegenerative diseases; however, the mechanisms of NFT formation remain unknown. Molecular studies of frontotemporal dementia with parkinsonism‐17 demonstrated that increasing the ratio of tau with exon 10 insertion induced fibrillar tau accumulation. Here, we show that carboxyl terminus of Hsc70‐interacting protein (CHIP), a U‐box protein, recognizes the microtubule‐binding repeat region of tau and preferentially ubiquitylates four‐repeat tau compared with three‐repeat tau. Overexpression of CHIP induced the prompt degradation of tau, reduced the formation of detergent‐insoluble tau and inhibited proteasome inhibitor‐induced cell death. NFT bearing neurons in progressive supranuclear palsy, in which four‐repeat tau is a component, showed the accumulation of CHIP. Thus, CHIP is a ubiquitin ligase for four‐repeat tau and maintains neuronal survival by regulating the quality control of tau in neurons.

Acceleration and persistence of neurofibrillary pathology in a mouse model of tauopathy following anesthesia

Alzheimers disease and other tauopa‐thies are characterized by the presence of intracellular neurofibrillary tangles composed of hyperphosphorylated, insoluble tau. General anesthesia has been shown to be associated with increased risk of Alzheimers disease, and we have previously demonstrated that anesthesia induces hypothermia, which leads to overt tau hyperphosphorylation in the brain of mice regardless of the anesthetic used. To investigate whether anesthesia enhances the long‐term risk of developing pathological forms of tau, we exposed a mouse model with tauopathy to anesthesia and monitored the outcome at two time points—during anesthesia, or 1 wk after exposure. We found that exposure to isoflurane at clinically relevant doses led to increased levels of phospho‐tau, increased insoluble, aggregated forms of tau, and detachment of tau from microtubules. Furthermore, levels of phospho‐tau distributed in the neuropil, as well as in cell bodies increased. Interestingly, the level of insoluble tau was increased 1 wk following anesthesia, suggesting that anesthesia precipitates changes in the brain that provoke the later development of tauopathy. Overall, our results suggest that anesthesia‐induced hypothermia could lead to an acceleration of tau pathology in vivo that could have significant clinical implications for patients with early stage, or overt neurofibrillary tangle pathology.— Planel, E.,Bretteville, A., Liu, L., Virag, L., Du, A. L., Yu, W. Y., Dickson, D. W., Whittington, R. A., Duff, K. E. Acceleration and persistence of neurofibrillary pathology in a mouse model of tauopathy following anesthesia. FASEBJ. 23, 2595–2604 (2009)

A transgenic rat that develops Alzheimer's disease-like amyloid pathology, deficits in synaptic plasticity and cognitive impairment

In the last decade, multiple lines of transgenic APP overexpressing mice have been created that recapitulate certain aspects of Alzheimers disease (AD). However, none of the previously reported transgenic APP overexpressing rat models developed AD-like beta-amyloid (Abeta) deposits, or age-related learning and memory deficits. In the present study, we have characterized a transgenic rat model overexpressing transgenes with three, familial AD mutations (two in APP and one in PS1) that were developed by Flood et al. [Flood, D.G., et al., Abeta deposition in a transgenic rat model of Alzheimers disease. Society for Neuroscience 2003, Washington, DC, 2003]. From the age of 9 months, these rats develop Abeta deposits in both diffuse and compact forms, with the latter being closely associated with activated microglia and reactive astrocytes. Impaired long-term potentiation (LTP) was revealed by electrophysiological recordings performed on hippocampal slices from rats at 7 months of age, which is 2 months before the appearance of amyloid plaques. The deficit in LTP was accompanied by impaired spatial learning and memory in the Morris water maze, which became more pronounced in transgenic rats of 13 months of age. For Tg rats of both ages, there was a trend for cognitive impairment to correlate with total Abeta42 levels in the hippocampus. The rat model therefore recapitulates AD-like amyloid pathology and cognitive impairment. The advantage of the rat model over the available mouse models is that rats provide better opportunities for advanced studies, such as serial CSF sampling, electrophysiology, neuroimaging, cell-based transplant manipulations, and complex behavioral testing.

Anesthesia-Induced Hyperphosphorylation Detaches 3-Repeat Tau from Microtubules without Affecting Their Stability In Vivo

In Alzheimers disease, tau is hyperphosphorylated, which is thought to detach it from microtubules (MTs), induce MT destabilization, and promote aggregation. Using a previously described in vivo model, we investigated whether hyperphosphorylation impacts tau function in wild-type and transgenic mice. We found that after anesthesia-induced hypothermia, MT-free tau was hyperphosphorylated, which impaired its ability to bind MTs and promote MT assembly. MT-bound tau was more resistant to hyperphosphorylation compared with free tau and tau did not dissociate from MTs in wild-type mice. However, 3-repeat tau detached from MT in the transgenic mice. Surprisingly, dissociation of tau from MTs did not lead to overt depolymerization of tubulin, and there was no collapse, or disturbance of axonal MT networks. These results indicate that, in vivo, a subpopulation of tau bound to MTs does not easily dissociate under conditions that extensively phosphorylate tau. Tau remaining on the MTs under these conditions is sufficient to maintain MT network integrity.

Untangling memory deficits

The buildup of protein aggregates consisting of proteins such as tau, huntingtin and amyloid-β occurs in a range of degenerative disorders. Recent studies have begun to shed light on the relative contribution of aggregates versus other, intermediate forms of the protein to disease. The most recent entry in the field examines the protein tau.

c-jun N-terminal kinase hyperphosphorylates R406W tau at the PHF-1 site during mitosis

Tauopathies such as Alzheimer disease (AD) probably involve a type of phosphorylation imbalance causing the accumulation of abnormally hyperphosphorylated tau in neurons and/or glias. Investigation of R406W tau mutation may provide insight into such abnormal tau hyperphosphorylation, since this mutation causes AD‐like dementia and tauopathy in humans and because it has the unique ability to reduce tau phosphorylation in vitro and in cultured cells. Here we show that R406W mutation primarily disrupts tau phosphorylation at Ser404, a priming phosphorylation site of glycogen synthase kinase‐3β (GSK‐3β), thereby reducing subsequent GSK‐3β‐mediated phosphorylation at the PHF‐1 site (mostly Ser396). In contrast, c‐jun N‐terminal kinase (JNK) as activated in the mitotic phase directly hyperphosphorylates R406W tau at the PHF‐1 site. This was confirmed by PHF‐1 hyperphosphorylation of R406W tau in mitotic cells, its association with cytoplasmic JNK activation, and its inhibition by a JNK inhibitor, SP600125. These data unveil the unknown mechanisms of physiological tau phosphorylation at the PHF‐1 site and suggest that cytoplasmic JNK activation may play an important role in the abnormal tau hyperphosphorylation associated with R406W tau mutation and in AD.